This invention generally relates to print head nozzle plates and methods and more particularly relates to a mandrel for forming an ink jet nozzle plate having orifices of precise size and location, and method of making the mandrel.
An ink jet printer produces images on a receiver by ejecting ink droplets onto the receiver in an imagewise fashion. The advantages of nonimpact, low-noise, low energy use, and low cost operation in addition to the capability of the printer to print on plain paper are largely responsible for the wide acceptance of ink jet printers in the marketplace.
In the case of "drop on demand" ink jet printers, a print head formed of piezoelectric material includes a plurality of ink channels, each channel containing ink therein. Each of these channels is defined by a pair of oppositely disposed sidewalls. Also, each of these channels terminates in a channel opening for exit of ink droplets onto a receiver disposed opposite the openings. The piezoelectric material possesses piezoelectric properties such that an electric field applied to a selected pair of sidewalls produces a mechanical stress in the sidewalls. Thus, the pair of sidewalls inwardly deform as the mechanical stress is produced by the applied electric field. As the pair of sidewalls defining the channel inwardly deform, an ink droplet is squeezed from the channel. Some naturally occurring materials possessing such piezoelectric characteristics are quartz and tourmaline. The most commonly produced piezoelectric ceramics are lead zirconate titanate (PZT), barium titanate, lead titanate, and lead metaniobate. However, it is desirable that the ink droplet exiting the channel opening travel in a predetermined trajectory so that the droplet has a predetermined velocity and volume and lands on the receiver at a predetermined location.
Therefore, it is customary to attach a nozzle plate to the print head such that the nozzle plate faces the receiver, so that the ink droplet achieves the desired volume and trajectory. The nozzle plate has nozzle orifices therethrough aligned with respective ones of the channel openings. The purpose of the orifices is to produce ink droplets having a predetermined volume and velocity. Another purpose of the orifices is to direct each ink droplet along a trajectory normal (i.e., at a right angle) to the nozzle plate and thus normal to the receiver surface. If diameter of the nozzle orifice deviates from a desired diameter, ink droplet trajectory, volume and velocity can vary from desired values. Moreover, deviation from desired values of trajectory, volume and velocity can occur if the nozzle orifice has an irregular, non-circular shape. Thus, such a nozzle plate should ensure that the ink droplet exiting the channel opening will travel along the predetermined trajectory with the predetermined volume and velocity so that the droplet lands on the receiver at the predetermined location and produces a pixel of predetermined size. To accomplish this result, each orifice is preferably precisely dimensioned so that each ink droplet exiting any of orifices travels along the predetermined trajectory with predetermined volume and velocity. This is important in order to avoid image artifacts, such as banding. Therefore, the technique used to make the nozzle plate should produce nozzle plate orifices that are precisely dimensioned and located to avoid such undesirable image artifacts.
Such a nozzle plate may be formed by a "negative relief" electroplating patterning process. In this process, a mandrel is formed by overcoating a substrate (e.g., silicon oxide or other nonconductive material) with a conductive film (e.g., chromium or nickel). A photoresist layer is then applied to the conductive film, which photoresist layer may be formed of sensitized resins or other suitable material. The photoresist layer is imaged and developed to expose selected areas of the conductive film. These selected exposed areas of conductive film are removed by exposing the film to an etchant, thereby leaving a relief pattern to complete formation of the previously mentioned mandrel. Such an etchant may be sodium hydroxide and potassium iron cyanate. Typically, the selected areas removed from the conductive film are circular holes, each hole corresponding to one of the nozzle orifices.
The nozzle plate itself may be formed by using the mandrel in combination with an electroplating process. In this regard, a layer of metal is electroplated over the conductive film and initially covers only the conductive film. Thereafter, the metal layer develops a growth front that closes over the circular holes where the conductive film was removed. The orifice diameter is defined by the edge of the growth front of the metal layer on the substrate. Thus, nozzle orifice diameter is determined by controlling the electroplating time. Alternatively, nozzle plates may be formed by an electroforming process using a mandrel having a "positive relief" pattern, such as caused by nonconductive disks on the conductive surface of the substrate, rather than the "negative relief" pattern mentioned hereinabove.
However, use of either the "positive relief" electroplating process or the "negative relief" electroforming process has various problems associated with it. A problem associated with each of these processes is variability of diameter of nozzle orifices. This may be due to the growth rate of the metal layer varying at different areas of the mandrel in the electroplating process (or electroforming process). Such variability in growth rate of the metal layer results in variability in diameter of the orifices, which diameter is defined by the previously mentioned growth front of the metal layer. Even relatively slight variability in growth rate of the metal layer in the electroplating (or electroforming) process can result in large relative error in orifice diameter. This problem is particularly severe when the techniques hereinabove are used to produce nozzle plates having small diameters which may be on the order of 10 .mu.m to 30 .mu.m. Thus, a problem in the art is variability in orifice diameter during manufacture of the nozzle plate.
Still another problem in the art is variability in nozzle orifice shape. That is, the prior art techniques mentioned hereinabove may sometimes produce noncircular orifices. This is undesirable because variability in orifice shape may also produce the previously mentioned image artifacts, such as banding. Such variability in orifice shape also may be due to uneven advancement of the metal layer growth front.
Yet another problem in the art is that some orifices may be formed completely closed. Of course, this is undesirable because completely closed orifices will produce the previously mentioned image artifacts, such as banding. Completely closed orifices may be due to completely uncontrolled advancement of the metal layer growth front.
Each of the problems identified hereinabove increases fabrication costs because each problem leads to rejection of nozzle plates as unusable. Hence, it is desirable to provide a nozzle plate having orifices of predetermined diameter and pitch in order to produce ink droplets of predetermined trajectory, volume and velocity.
Therefore, what is needed is a mandrel for forming an ink jet nozzle plate having orifices of precise size and location, and method of making the mandrel.